1. Field of the Invention
The present invention generally relates to spinal fixation systems and the like. An embodiment of the present invention relates to a sacral screw used during procedures for stabilizing a human spine.
2. Description of the Related Art
Spinal fixation procedures, such as lumbar sacral fusion and the correction of spinal deformities such as scoliotic curves, are well known and frequently used medical procedures. Pedicle, lateral, and oblique mounting devices may be used to secure corrective spinal instrumentation to a portion of the spine that has been selected to be fused by arthrodesis.
A spinal fixation system typically includes corrective spinal instrumentation that is attached to selected vertebrae of the spine by screws, hooks, and clamps. The corrective spinal instrumentation includes spinal rods or plates that are generally positioned parallel to the patient""s back. The corrective spinal instrumentation may also include transverse connecting rods that extend between neighboring spinal rods. Spinal fixation systems may be used to correct problems in the lumbar and thoracic portions of the spine. Spinal fixation systems may be installed posterior to the spine on opposite sides of the spinous process and adjacent to the transverse process.
Various types of screws, hooks, and clamps have been used for attaching corrective spinal instrumentation to selected portions of the patient""s spine. Examples of screws and other types of attachments are illustrated in U.S. Pat. Nos. 4,763,644; 4,805,602; 4,887,596; 4,950,269; 5,129,388; and 5,743,914. Each of these patents is incorporated by reference as if fully set forth herein.
Spinal fixation procedures that involve fixation of the sacrum may be difficult. Sacral fixation procedures that require long constructs, and/or sagittal plane realignment or revision may be particularly troublesome. Also troublesome are sacral fixation procedures in patients who have weak bone. Some of the problems associated with sacral fixation procedures are the difficult anatomy of the area; the poor bone quality frequently found in the sacrum; and the large lumbosacral loads and cantilever pullout forces applied across the region. The bone quality of the sacrum may be poor even in patients who do not have weak bone.
One sacral fixation procedure involves the insertion of spinal rods into the sacrum. The procedure is described in a paper titled:xe2x80x9cThe xe2x80x98Sacroiliac Buttressxe2x80x99 Intrasacral And Translation-Rotation Force Application With Spinal Instrumentation,xe2x80x9d by Roger P. Jackson, M.D., and is incorporated by reference as if fully set forth herein.
A sacral screw assembly that may include a spinal rod, a fixation component, a connector, and a fastener. The fixation component may have a top section and a shank. The top section may have a cavity defined by a base and two walls. The cavity may narrow in a direction from the base toward a top of the cavity. The base may have a rod groove that complements the shape of the spinal rod and enables the sacral screw assembly to have a low profile above the sacrum during use. The fixation component may include indentations in the outer surfaces of the walls. An indentation may allow instrumentation to properly position a connector within a cavity of a fixation component. Upper ends of walls of the fixation component may form semi-circular or arcuate wedges.
The shank of the fixation component may have a variable diameter and a tapered end. The diameter of the shank may be widest at a location on the shank adjacent to the upper section of the fixation component. Alternatively, the shank may have a constant width and a tapered end. The shank may be threaded. The outer diameter of the thread may widen as the thread approaches the upper section of the fixation component. In another embodiment, the spinal fixation component has two flights of thread. The first thread extends from a position near the end of the shank to a position near the upper section of the fixation component. The second thread begins near the end of the shank, but the thread terminates a significant distance below the base. The combination of two flights of threads on the shank produces a fine pitch thread section and a coarse pitch thread section. The fine pitch thread section provides good purchase in dense bone material, and the coarse pitch thread section provides good purchase in less dense bone material. The combination of a fine pitch thread section and a coarse pitch thread section may provide the bone screw with increased resistance to thread backout.
The connector may include an upper section and a lower section. A portion of the upper section of the connector may be threaded. A threaded section of the fastener may engage the thread of the connector to secure the connector to the fixation component during use. The bottom of the fastener may have a ring that engages the wedges on the fixation component. The ring and wedges may ensure that the connector is properly positioned within the fixation component. The ring and wedges may also prevent the walls of the fixation component from flaring outwards when the fastener attaches a connector to the fixation component. The top of the connector may be rounded so that the top of the connector generally conforms to the shape of the fastener when the fastener attaches the connector to the fixation component. A rounded top of the connector may ensure that no sharp edges are present at the interface between the connector and the fastener when the fastener attaches the connector to the fixation component.
The lower section of the connector may include two arms that define an opening. A spinal rod may be inserted into the opening of the connector. A slot may be located in the upper section of the connector between the two arms. The slot may allow the arms to be deflected relative to each other. When the slot narrows, the arms apply a compressive force against a rod positioned in the opening to securely hold the rod. When the slot widens, a rod positioned within the opening may be positioned or removed from the opening.
The connector may be configured to be at least partially disposed within the cavity of the fixation component during use. The inner surfaces of the cavity walls may exert a compressive force onto outer surfaces of the connector. The compressive force exerted on the outer surfaces of the connector may serve to narrow the slot in the upper section of the connector. Narrowing the slot may inhibit movement of a spinal rod positioned within the opening of the connector. The surface of the spinal rod to which the connector attaches and/or surfaces of the opening, may be textured to further reduce the possibility of movement of a spinal rod positioned within the opening.
The arms may have flared portions at the ends of the arms. The cavity of the fixation component may have grooves configured to receive the flared portions of the arms. The flared portions of the rods may allow for the easy placement of a connector within the cavity of a fixation component. The flared portions of the rods may also inhibit displacement of the connector through the top of the cavity prior to securing the connector to the fixation component with the fastener. An outer surface of one or both of the arms of the connector may taper to complement the narrowing of the cavity from the base to the top of the cavity. The surfaces of the cavity walls and/or outer surfaces of the arms may be textured.
The sacral screw assembly may be inserted into a sacrum. In one procedure, the threaded shank of a fixation component is screwed into the sacrum at a desired location. A connector is snapped onto a spinal rod, and the connector is properly positioned within the cavity of the fixation component. The open top of the fixation component allows good visibility of the surgical site during placement and positioning of spinal rod and connector. A fastener may then be threaded onto the connector to attach the connector to the fixation component. Tightening the fastener securely fixes the position of the spinal rod relative to the fixation component, and tightening the fastener inhibits translational and rotational motion of the spinal rod. A second connector may be snapped onto the rod. The second connector may be used to attach the rod to a second fixation component or to a fixation device, such as a hook. The second connector may be positioned without detaching the rod from the fixation component, and without altering the position of any other connectors, fixation components, or fixation devices attached to the rod.